Overstrength Factor in Anchor Design for Seismic Loading - Vertical Process Vessel

[bones206]

There is a bit of debate in my office and I am currently in the minority. My boss and others think that we should design the anchor bolts for a 90 ft tall process vessel (SDC C) using the overstrength factor option of ACI 318 Appendix D/Ch17. My understanding is that the overstrength method of designing anchors for this specific situation is prohibited by ASCE 7-10, Section 15.7.5:

15.7.5 Anchorage
Tanks and vessels at grade are permitted to be designed without anchorage where they meet the requirements for unanchored tanks in reference documents. Tanks and vessels supported above grade on structural towers or building structures shall be anchored to the supporting structure. The following special detailing requirements shall apply to steel tank and vessel anchor bolts in SDC C, D, E, and F. Anchorage shall be in accordance with Section 15.4.9, whereby the anchor embedment into the concrete shall be designed to develop the steel strength of the anchor in tension. The steel strength of the anchor in tension shall be determined in accordance with ACI 318, Appendix D, Eq. D-3. The anchor shall have a minimum gauge length of eight diameters. Post-installed anchors are permitted to be used in accordance with Section 15.4.9.3 provided the anchor embedment into the concrete is designed to develop the steel strength of the anchor in tension. [highlight #FCE94F]In either case, the load combinations with overstrength of Section 12.4.3 are not to be used to size the anchor bolts for tanks and horizontal and vertical vessels.[/highlight]

The underlying philosophy as I understand it is to force the anchors to be the ductile "fuse" of the foundation/anchorage system in order to promote bolt yielding, which absorbs the seismic energy hysterically and helps avoid a brittle-type failure or foundation overturning. I've read the ASCE

Does anyone know if there is an alternative approach where the overstrength factor method can be utilized, that is also code compliant? In other words, am I wrong or is my boss wrong? Either way I lose

 

[sandman21]

The commentary for the section

Many steel tanks can be designed without anchors by using annular plate detailing in accordance with reference documents. Where tanks must be anchored because of overturning potential, proper anchorage design provides both a shell attachment and an embedment detail that will allow the bolt to yield without tearing the shell or pulling the bolt out of the foundation. Properly designed anchored tanks have greater reserve strength to resist seismic overload than do unanchored tanks. To ensure that the bolt yields (stretches) before failure of the anchor embedment, the anchor embedment must be designed in accordance with ACI 318, Appendix D, Equation D-3 and must be provided with a minimum gauge length of eight bolt diameters. Gauge length is the length of the bolt that is allowed to stretch. It may include part of the embedment length into the concrete that is not bonded to the bolt. A representation of gauge length is shown in Figure C15.7-1. It is also important that the bolt not be significantly oversized to ensure that the bolt stretches. The prohibition on using the load combinations with overstrength of Section 12.4.3 is intended to accomplish this goal.

 

[Rabbit12]

I agree with Sandman21. Design the anchor for full tensile strength. No need to use the overstrength factor. It'll likely take reinforcing designed to carry the anchor load. I've used the paper "Design of Anchor Reinforcement in Concrete Pedestals" by Widianto, Patel, and Owen.

 

[bones206]

Ok, but do you agree that the overstrength factor is explicitly prohibited unless the vessel is SDC B?

 

[sandman21]

Section 15.7.5 for the anchorage of tanks, specifically prohibits the use of overstrength. ACI Ch. 17 has four methods to satisfy 17.2.3.4.3. Section 15.7.5 for the most part is following option a. As a compromise design the anchorage per 17.2.3.4.3(a)

 

[bones206]

Thanks -- that's been my argument as well, but I wanted to throw the question out there to make sure I wasn't missing some other loophole or misinterpreting something in the code.

 

[bones206]

So the only time overstrength factors listed for tanks and vessels are used is for the design of connecting elements like anchor chairs, or compression struts in support towers (as per 15.7.3), correct?